When you approach the aperture to the southwest, and press the switch there, which will reveal a cyberdemon as well as a passage behind him. Go through this passageway, but turn around to find that a hill to the northwest has opened up. Maneuver through the trees until you find a switch. Use it and head back southeast to find a megaarmor. (sector 1673)

From Secret #3, go along the second step in the other direction until you reach a skull switch. This will lower the BFG9000 to the northwest. (sector 1758)

As you head to the northeast portion of the map, you will notice a soul sphere on the column. Keep going along the path until you reach a dropoff point just past the arch-vile, where you can get on the base of the column. Follow this base all the way around until you reach the soul sphere. (sector 2134)